Color proofs are used by the graphic arts industry, usually prior to printing, to determine how a printed color image will appear. Proofs provide an inexpensive way to check the quality of color separations prior to an actual printing run.
Many photosensitive elements are known which are useful for the preparation of color proofs. After imagewise exposure the photosensitive elements may be developed by organic solvent or aqueous washout of soluble image areas, toning tacky image areas with a colorant, peeling apart photoadherent layers, or combinations of these techniques. A useful reference for color proofing methods is Principles of Color Proofing, by Michael H. Bruno, GAMA Communications, Salem, NH, 1986.
Clear, uncolored, photopolymerizable or photohardenable layers may be developed after imagewise exposure by applying colorants which adhere selectively to the unexposed areas. Chu et al., U.S. Pat. No. 3,649,268, describes a process in which an element having a removable support and a photohardenable layer is (1) laminated to a receptor layer, and (2) imagewise exposed using a color separation transparency of the original image to be reproduced through the support to actinic radiation to selectively raise the tack temperature of the areas receiving the radiation, then (3) the support is stripped from the layer, and the outer surface of the layer treated (e.g, dusted) with a particulate material, usually a colored pigment, or toner, which adheres only to the tacky unexposed areas of the layer thereby producing a positive image. By laminating additional elements to the toned surface, exposing each element with a different color separation record, stripping, and toning with a corresponding color, a multicolor image can be obtained. However, the process has the limitation of producing only a positive-working or duplicate image (i.e., the pigment image is a duplicate of the positive transparency used for exposure) and is, therefore, restricted in use, particularly where it is desired to produce a negative or reverse image (i.e., an image which is complementary to the image on the transparency used for exposure). This process also requires that the image be developed by application of a colorant.
Dueber et al., U.S. Pat. No. 4,477,556, the teaching of which is incorporated by reference, describes compositions and processes which are capable of producing either a positive or negative polymeric image on a substrate. The photopolymerizable layer contains a photosensitive acidic o-nitroaromatic compound which generates a polymerization-inhibiting nitrosoaromatic compound. However, the development process of Dueber et al. has the disadvantage of being a washout procedure, i.e., one which uses liquids, rather than a dry process.
In Cohen and Fan, U.S. Pat. No. 4,282,308 there is described a photopolymerizable element which is capable of producing reverse colored images by a dry process without the need for toning with a colorant. The element comprises, in order from top to bottom, a transparent strippable cover sheet, a photoadherent layer containing a colorant, a tacky nonphotosensitive organic contiguous layer, and a support. After imagewise exposure to actinic radiation through the cover sheet, the element can be peeled apart by peeling off the cover sheet with the exposed areas of the photoadherent layer adherent thereto, thus forming a negative image on the cover sheet. The peel force required to remove the cover sheet and the exposed photoadherent layer from the contiguous layer is at least four times the peel force required to remove the cover sheet from an unexposed photoadherent layer. When the exposure is through a negative image, e.g., a negative separation transparency, and the element is peeled apart, a positive image is obtained on the peeled off cover sheet and a negative image on the contiguous layer. When the exposure is through a positive image, e.g., a positive separation transparency, and the element is peeled apart, a negative image is found on the cover sheet and a positive image is obtained on the supported tacky contiguous layer. To produce a negative multicolor proof, a series of appropriately colored negative images would have to be laminated in register on top of each other.
In Taylor, U.S. Pat. No. 4,489,154, the disclosure of which is incorporated herein by reference, there is described a process for preparing a surprint proof by a dry process without the need for toning with a colorant. This process uses an imagewise exposed, peel-apart photosensitive element comprising a strippable cover sheet; a photoadherent layer containing a colorant; a tacky, essentially nonphotosensitive contiguous layer; and a support. After exposure to actinic radiation through a positive image, e.g., a positive separation transparency, and peeling apart, a positive colored photoadherent image is produced on the supported contiguous layer. This image is then adhered to a substrate. By repeating this process with different colored layers, a multicolor surprint proof can be built up on the substrate. If exposure is through a negative image, e.g., a negative separation transparency, a positive colored photoadherent image is obtained on the cover sheet. While this image can be transferred to a substrate and the process repeated as above to build up a negative or reverse surprint proof, i.e., a positive proof produced from a set of negative separation transparencies, it is generally not practical to do so because of the problems of registration and image distortion which result from laminating after the exposure step.
It is desirable to provide an improved method of producing a negative or reverse surprint proof. Such a method should be a dry process which also does not require application of a colorant, and does not require lamination after exposure. It is also desirable to provide an improved proofing method utilizing elements which can produce either a positive or a negative surprint proof.